When I was a kid, we'd go to the beach
and dig in the sand. We caught tiny crab-like creatures that scurried and dug burrows right at the edge of the surf. They never bit or pinched, and they were cute and fun to play with. We called them sand crabs
After I'd grown up, I walked down different beaches and saw fishermen dragging cage-like metal contraptions across the sand. They dumped out their catch, which they called sand fleas, in buckets to use as bait for their hooks. These were the same tiny animals I'd dug up with my cousins years ago.
It turns out that these little crabs are crustaceans of genus Emerita. There seem to be several species dispersed across the world's beaches; two of the more common species are Emerita talpoida on the Atlantic coast of North America, and Emerita analoga on the Pacific shores. They're not true crabs, so to speak, since they walk backwards rather than sideways.
There doesn't seem to be a general consensus about what to call these things. Mole crab seems to be a common name for them, but this name is also used for several varieties of "real" crabs. The same problem exists for the name of sand crab. Sand flea might be good, but they're not really fleas, of course, and that name is used properly for certain blood-sucking insects that are.
Whatever you want to call them, they're exceedingly common on many sandy beaches. Moving up and down the beach to keep with the tide, they bury themselves in the sand and eat tiny flecks of organic material drifting in the water. They grow up to a few centimeters in length, depending on species, with females usually about twice the size of males.
The sand crab's compact body and tough outer shell protect it well from the constant tumbling action of the waves, but not from the multitude of birds and fish that eat it. Those I've seen are colored a nondescript beige and blend in well with the sand; I suppose this is true throughout the rest of the world as well. Their eggs are a bright and very noticeable orange, though.
Although I've never done any surf fishing, those that do unilaterally declare sand crabs (usually sand fleas in this context) to be excellent bait. It's an ideal situation, really - where else can you catch your bait (and know it's fresh) moments before casting your line? Fish apparently find them delicious enough that fishermen in areas where sand fleas are uncommon will sometimes buy sand flea extracts and other such concoctions to make their bait more appealing to the fish they're trying to catch.